I've have been building a Multi-App. The app with the most code is Paint Pot. As the image shows. It was complicated to make this PAINT POT. The reason for why it was difficult was that half of the time MY WORK GOT DELETED AND I HAD TO REDO IT.AND IT HAD MANY CODES YOU HAD TO DO FOR EVERY BUTTON TO WORK. :(

 So there were many hardships when I made this app. But I believe that I finally completed Paint Pot. And also I made this app with MIT App Inventor